Certain types of electronic systems include isolation circuits in their communications paths to achieve signal compatibility, provide protection from excessive voltage, and reduce signal noise. Industrial equipment and automotive electronics are two such systems. These systems often include a processor or controller that receives data from sensors and other circuits and sends data to control circuits and equipment such as motors and valves. Isolation circuits provide the communication paths between these components. Isolation circuits are useful to simplify system implementation where the circuits sending and receiving data use different power supplies. Isolation circuits typically include isolation barriers as part of the approach to reduce electrical interaction between the circuits sending and receiving data. These isolation barriers often include components, such as capacitors, transformers and optical devices. These components provide a path for signals to cross the isolation barrier without direct conductive coupling, such as with wires, circuit board traces, and integrated circuit metallic interconnect.
Isolation circuits often form a portion of the lowest layer in a so-called protocol stack. This lowest level is known as the physical layer. The lowest layer of the protocol stack typically provides the electronic path that transfers data bits (in the case of digital communications) from the input of the path to the output of the path. For digital communications, voltage levels or voltage transitions often represent data bits. Some systems employ current loops or other approaches to transfer data. Higher layers of the protocol stack may organize data bits into structures, such as packets and frames. Packets and frames may include header data bits, data payload bits, and error detection bits. Isolation circuits typically operate independently from the higher levels of the protocol stack, and thus transfer data bits regardless of the packet or frame structure and content. Communications in systems that do not use structures (such as packets and frames) can also use isolation circuits, which are further useful in systems that do not use protocol stacks.